Preheat the Oven and Prepare the Pan: Preheat the oven to 350°F (175°C). Position a rack in the center.

Lightly grease a 9 × 5-inch loaf pan and line the bottom and long sides with parchment paper, leaving enough overhang to lift the baked bread from the pan.

Prepare the Cinnamon Swirl: In a small bowl, combine the brown sugar, cinnamon, and flour for the swirl.

Add the softened butter and mash with a fork until the mixture resembles thick, moist crumbs.

It should hold together when pressed but should not be runny. Set aside.

Mash the Bananas: Peel the ripe bananas and mash them in a large mixing bowl until mostly smooth.

A few small lumps are fine, but avoid leaving large chunks that can create wet pockets in the baked loaf.

Measure exactly 1½ cups (340 g) of mashed banana.

Mix the Wet Ingredients: Add the granulated sugar and brown sugar to the mashed banana and whisk until combined.

Add the eggs, vegetable oil, whole milk, and vanilla extract. Whisk until the mixture looks smooth and evenly blended.

Combine the Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, kosher salt, and cinnamon.

Mix thoroughly so the leavening agents are evenly distributed throughout the flour.

Make the Banana Bread Batter: Add the dry ingredients to the wet mixture. Fold gently with a rubber spatula until no visible streaks of flour remain.

The batter should be thick but easily spreadable: Stop mixing as soon as everything is combined to avoid developing excess gluten.

Add the First Layer: Spread approximately half of the banana bread batter evenly in the prepared loaf pan.

Use a spatula to push the batter gently into the corners and smooth the surface.

Add the Cinnamon Filling: Reserve about 2 tablespoons of the cinnamon mixture for the top.

Sprinkle the remaining cinnamon mixture evenly over the first layer of batter, keeping it about ½ inch away from the edges of the pan.

This helps prevent the sugar filling from sticking to the sides.

Add the Remaining Batter and Swirl: Carefully spread the remaining banana batter over the cinnamon layer.

Sprinkle the reserved cinnamon mixture over the surface: Insert a butter knife or wooden skewer into the batter and make two or three gentle S-shaped movements through the loaf.

Avoid over-swirling, which can blend the cinnamon mixture completely into the batter instead of leaving distinct ribbons.

Bake the Bread: Bake at 350°F (175°C) for 55–65 minutes. The loaf should rise, develop a deep golden-brown top, and begin pulling slightly away from the sides of the pan.

Insert a toothpick or wooden skewer into the banana bread portion near the center. It should come out clean or with a few moist crumbs, but no raw batter.

Avoid testing directly through a cinnamon pocket, which may look wet even when the bread is fully baked.

If the top browns too quickly, loosely cover it with aluminum foil during the final 15 minutes.

Cool the Loaf: Remove the pan from the oven and place it on a wire rack.

Let the bread cool in the pan for 15 minutes.

Use the parchment overhang to lift the loaf out, then place it directly on the rack and cool for another 30 minutes before glazing.

The loaf should be warm or completely cool rather than hot so the glaze stays visible instead of melting into the crust.

Make the Vanilla Glaze: In a small bowl, whisk together the sifted powdered sugar, whole milk, and vanilla extract.

The glaze should be smooth and thick enough to fall from the whisk in slow ribbons.

If it is too thick, add a few drops of milk. If it becomes too thin, whisk in a small amount of additional powdered sugar.

Glaze and Serve: Drizzle the vanilla glaze across the cooled loaf. Allow the glaze to set for several minutes, then cut the bread into 10 slices with a serrated knife.

The finished loaf should have a soft banana crumb, distinct cinnamon-sugar ribbons, and a lightly sweet vanilla finish.